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Ice Motion from Passive Microwave: SMMR, SSM/I, SSMIS, and AMSR-E

Methods

The SMMR, SSM/I, SSMIS, and AMSR-E data are reprojected from a polar stereographic projection to a corresponding 25 km EASE-Grid projection. All images are 24-hour composites of all available passes except for SMMR which only has data available every 48 hours. See the Accuracy section below for a discussion of the consequences of this. The images cover sea ice areas of both polar regions completely.

The SMMR, SMM/I, SSMIS, and AMSR-E passive microwave instruments have differing channel frequencies, resolutions, and temporal coverage. See Table 11 in the main ice motion documentation for frequencies and resolutions and Table 8 for temporal coverage of the instruments. Detection of ice displacement in passive microwave data were achieved with maximum cross correlation (MCC) techniques described in Emery et al. (1995). The MCC methods were applied to the two SMMR images (37H GHz and 37V GHz channels), the four SSM/I images (37H GHz, 37V GHz, 85H GHz, and 85V GHz channels), the four SSMIS images (37H GHz, 37V GHz, 91H GHz, and 91V GHz channels), and one AMSR-E image (89V). Fowler compared 10 x 10 pixel rectangular subsets of the same spatial locations between two consecutive days and chose the location with the best correlation coefficient. The change in location is considered the ice displacement, which allows ice motion to be calculated. This method applies to each of the channels for each of the four passes.

Because there are two different channels, a higher frequency and lower frequency, used from the SSM/I and SSMIS instruments, the vectors from the two channels are merged together, with vectors available for both frequencies in most cases. The precision of the vectors from the 85 GHz channel is better than that from the 37 GHz channel; however, periods of high water vapor affect 85 GHz more than 37 GHz; so the ability to track ice is reduced. Vectors from 85 GHz are preferable. In cases where 85 GHz and 37 GHz vectors overlap, the 37 GHz vector is removed .

The following image shows ice motion from SSM/I on 20 April 2000.

Daily-averaged ice motion vectors from SSM/I

Accuracy

Direct comparisons between the more accurate buoy measurements and vectors derived from passive microwave data are difficult due to time differences. The SMMR data are composites over two days, and the data from the other sensors are composites over a 24-hour period. Each pixel in an image may contain information from more than one orbit, and each pixel represents the composite motion of ice within the pixel area, while buoy motion tracks the drift of a specific ice floe or patch of ice. With these problems in mind, vectors derived from passive microwave imagery were compared with those from buoy data.

For 85 GHz data, there were 74,381 pairs of buoy and SSM/I vectors that were less than 50 km apart. The mean difference in the u component was -0.05 cm/sec with a Root Mean Square (RMS) error of 4.16 cm/sec. The mean difference in the v component was 0.39 cm/sec with an RMS error of 4.23 cm/sec.

For 37 GHz data, the mean difference in the u component was 0.04 cm/sec with a Root Mean Square (RMS) error of 5.05 cm/sec. The mean difference in the v component was 0.74 cm/sec with an RMS error of 5.24 cm/sec.

The pixel resolution of the AMSR-E data is similar to the resolution of the AVHRR data, and the comparison with buoys showed much the same error, with a mean error of 0.19 cm/sec and root-mean-sqaure error of 3.52 cm/sec.

SMMR 48 Hour Temporal Resolution Consequences

The passive microwave motion estimates using SMMR are made using data separated by 48 hours while SSM/I-SSMIS data are 24 hours apart. SMMR data were used from 25 October 1978 through 08 July 1987 when the daily SSM/I data became available.

The daily speeds estimated by SSM/I-SSMIS data appear to be higher than SMMR data, but this is likely primarily explained by the fact that sampling every 24 hours instead of every 48 hours with the same grid spacing yields discrete speed estimates. Two day averages of the net 37 GHz SSM/I ice speeds are comparable to the 48-hour speeds computed from SMMR. However, the motion estimates derived from the horizontal and vertical polarization channels of the higher resolution channel, indicated by a value of 3.0 in the 5th column of the data file, have been found to have larger average speeds than those derived from the 37 GHz channels. This is likely primarily explained by the difference in spatial sampling relative to the time interval.

While these sensor differences affect daily motion estimates in the raw passive microwave fields, average drift aggregated over many motion estimates is largely unbiased. Therefore, these effects are ameliorated in the gridded interpolated files and are minimal in the weekly and monthly average fields.